1. Field of the Invention
The present invention relates to a micro-electromechanical device using a piezoelectric actuator.
2. Description of the Related Art
Recently, an actuator fabricated by micro-electromechanical system (MEMS) technology has attracted attention. In such an actuator, a beam of the actuator is bent and displaced by a driving force, such as an electrostatic force, a thermal stress, an electromagnetic force, and a piezoelectric force.
Micro-electromechanical devices, such as variable capacitors and switches, which use an actuator, have been proposed. A variable capacitor or switch fabricated by MEMS technology includes a movable electrode provided on a beam of the actuator having an end suspended over the free space on a substrate, and a fixed electrode provided on a surface of the substrate. The movable electrode and the fixed electrode face each other in a direction perpendicular to a surface of the substrate. The actuator is bent so as to vary in distance between the movable electrode and the fixed electrode.
In particular, in a MEMS variable capacitor having a piezoelectric actuator which uses an inverse piezoelectric effect or an electrostrictive effect as a driving force, a movable beam of the piezoelectric actuator may continuously and widely vary an interval between a movable electrode and a fixed electrode. Accordingly, a capacitance variation of the MEMS variable capacitor may increase. Moreover, since air or gas between the movable electrode and the fixed electrode are used as a dielectric, the MEMS variable capacitor advantageously has an extremely large Q value and the like.
Moreover, the structure of a MEMS variable capacitor may also be applicable to a MEMS switch. For example, in a capacitive type switch, a movable electrode is capacitively coupled with a fixed electrode across an extremely thin dielectric film. Alternatively, in a DC contact type switch, a movable electrode is brought into direct contact with a fixed electrode. Such a switch fabricated by MEMS technology has also attracted attention, since the switch has both a low on-state resistance and a high isolation capability in an off-state.
A piezoelectric actuator has a long and thin beam including a piezoelectric film sandwiched by top and bottom electrodes. The beam is suspended over the free space on a substrate by fixing an end of the beam on a substrate. Therefore, it is a serious problem that the beam is vertically warped due to as light residual stress in a material of the piezoelectric film. Hence, it is extremely difficult to adjust a capacitance value with an applied voltage to a MEMS variable capacitor as designed, or to set a drive voltage of a MEMS switch at a steady value.
For example, at an operation end in which the movable electrode is provided in the piezoelectric actuator, a displacement D of the beam due to the electrostrictive effect is approximated by the following expression:D˜E·d31·L2·t−1  (1)where E is an electric field applied to the piezoelectric film, d31 is a piezoelectric strain coefficient of the piezoelectric film, and L and t are a length and thickness of the actuator, respectively.
A warpage Dw of the piezoelectric actuator, which is caused by a residual stress Sr occurring on the deposited piezoelectric film and the like, is approximated by the following expression:Dw˜Sr·L2·t−1  (2)
As understood from a comparison between expression (1) and expression (2), both of the displacement D and the warpage Dw have a similar relation regarding the length L and the thickness t of the piezoelectric actuator. Specifically, the displacement D and the warpage Dw are proportional to a square of length L, and are inversely proportional to the thickness t. For example, in order to increase a drive range of the piezoelectric actuator, it is effective to increase the length L or to decrease the thickness t. In response, the displacement D may be increased, but also the warpage Dw is increased. Hence, in order to increase the drive range of the piezoelectric actuator while suppressing the warpage, geometrical modification for the actuator may have almost no effect. There will be no other way but to reduce an absolute value of the residual stress Sr compared to an absolute value of the piezoelectric strain (E·d31) due to the electrostrictive effect.
With respect to lead zirconate titanate (PZT) that is a piezoelectric material having a large electrostrictive effect, it is necessary to anneal a PZT film at about 600  C. after the PZT film is deposited at room temperature in order to obtain a good film quality. A contraction in volume may occur due to such annealing. Accordingly, a residual stress of the PZT piezoelectric film is inevitably increased. In addition, for a piezoelectric film, such as aluminum nitride (AlN), zinc oxide (ZnO), and the like, which can be deposited at around room temperature with a good film quality, it is possible to relatively precisely control residual stress by adjusting deposition conditions. However, an electrostrictive effect in the piezoelectric film, such as AlN, ZnO, and the like, is smaller than the PZT film by a factor of ten or more.
In the case of using a piezoelectric material having such a large electrostrictive effect in order to increase the piezoelectric strain of the piezoelectric film of the piezoelectric actuator, it is difficult to control residual stress in a piezoelectric film, and to suppress warpage of the actuator. Moreover, a piezoelectric material, in which the residual stress can be controlled relatively easily, has a small electrostrictive effect. In such piezoelectric material, a drive range of the actuator cannot be sufficiently increased in comparison with the warpage of the actuator. Due to the problems as described above, technological application of the piezoelectric actuator is precluded. The piezoelectric actuator is strongly warped by the slight residual stress due to a serious problem relating to such a structure of the piezoelectric actuator, that is, the thin and long beam structure. Therefore, it is difficult to fabricate a MEMS variable capacitor with a constant capacitance, or a MEMS switch with a constant operation voltage.
The present inventors have proposed an actuator having a folded beam structure, as a measure for reducing warpage due to a residual stress of a piezoelectric actuator (refer to Japanese Patent Laid-Open Application No. 2006-87231). A piezoelectric actuator having a folded beam structure includes first and second beams. This structure is obtained by folding the second beam, which is substantially identical in shape and dimension to the first beam, with respect to a first beam that is fixed at a fixed end. A working end of the second beam is placed adjacent to the fixed end. In this way, the warpage of the first beam may be cancelled by the warpage of the second beam. Moreover, by applying a drive voltage to either one of the first and second beams, or by applying opposite drive voltages to each of the first and second beams, a drive displacement due to the piezoelectric action is not be canceled. Thus, a sufficient drive range may be ensured.
Based on the proposal of the inventors, design trials of the piezoelectric actuator have been performed. As a result, it has been proved that, although the warpage in a longitudinal direction in which the beams are extended is very effectively cancelled, the warpage caused by the distance in a lateral direction in which the fixed end and the working end face each other still exists. The warpage in the lateral direction depends on a ratio between a length and a width of the piezoelectric actuator. However, in so far as the trials, the warpage in the lateral direction is about 10% of the maximal warpage at the folded point of the piezoelectric actuator.
The warpage in the lateral direction of the piezoelectric actuator is nearly equal to the drive range of the piezoelectric actuator. Therefore, it is difficult to control the displacement accurately and repeatedly, with the piezoelectric drive. In this way, the warpage in the lateral direction, due to the residual stress of the piezoelectric actuator, decreases the manufacture yield and performance of a micro-electromechanical device using the piezoelectric actuator.